This invention relates to an electrically conductive rigid polyurethane foam.
There are many polymeric materials in existence which are electrically conductive, often due to the incorporation of electrically conductive fillers. Included among these known materials are various carbon-filled polyurethane foams, such as the well known series 1900 VELOSTAT foams. However, all of these electrically conductive polyurethane foams are flexible.
For example, U.S. Pat. No. 3,406,126 discloses solid casting resins such as epoxy or fiberglass, made electrically conductive by incorporation therein of unwoven carbon yarn filaments. Such composites are useful in discharging static electricity buildup, for example. U.S. Pat. No. 3,235,772 also deals with solid elastomers useful as anti-static printer's blankets. The elastomer layer may comprise flexible polyurethane and is filled with conductive carbon black. U.S. Pat. No. 4,351,745 also relates to a solid elastomer filled with commercial conductive carbon black fillers. U.S. Pat. Nos. 4,256,800 and 3,367,851, deal only with the application of conductive coatings via aqueous media. U.S. Pat. No. 4,286,004 also deals with flexible polyurethane foams as anti-static carpet backing. On the other hand, U.S. Pat. No. 3,208,013 does relate to rigid polyurethane foams. However, the resultant composition is an insulator.
In general, these electrically conductive flexible polyurethane foams are made by a multi-step process, typically involving a leaching out of the foam cells with a caustic material followed by the application of a conductive coating to the remaining polymer struts. However, such methodology is not applicable to rigid polyurethane foams because of the closed nature of their cell structure.
There is also a large body of literature relating to the general use of materials such as carbon blacks, metal-coated substrates such as glass beads, etc. as fillers for polymeric materials whereby the latter are rendered conductive. However, in no case has this been attempted with rigid polyurethane foams. See, e.g., Bigg, Polymer Engineering and Science, December 1977, Volume 17, No. 12, 842-847; Bodnar et al., SPE, 37th Annual Tech. Conference, New Orleans, LA, May 7-10, 1979, published by SPE, Greenwich, Conn., 1979, pp. 762-765; Bodnar et al., Mod. Plast., Volume 57, January 1980, 111-120; Theberge et al., American Chemical Society, Division of Org. Coat. Plast, Chem. Prepr., Volume 38, 175th National Meeting of ACS, Anaheim, CA, Mar. 12-17, 1978, published by American Chemical Society, Div. Org. Coat. Plast. Chem., Washington, D.C., 1977, 430-433; McGraw, H. R. and Hood, W. L., Mound Laboratory, Miamisburg, Ohio, Dec. 30, 1975, Contract: E-33-1-GEN-53, MLM-2192, NTIS; and Galli, Plastics Compounding, March/April 1982, 22-32.
In very many of these applications, serious problems can be encountered in the incorporation of a given filler in a given polymer, often because of an undesired deterioration of mechanical and other important polymer properties. Hence, there is no substantial predictability a priori of the acceptability of the incorporation of such a filler in a given polymer.
Although the electrically conductive flexible polyurethane foams are adequate for many applications, there are significant situations in which they are not sufficient and in which rigid polyurethane foams are needed. One important such application involves the use of rigid polyurethane foam in molded form as structural packaging material for sensitive electrical components. In conventional applications, e.g., computer-related products, such packaged components are not susceptible to electrostatic damage since the individual components form a large assembly and constitute one or more major circuits, each of which is grounded to the cabinet or chassis of the device. However, there are other applications, e.g., weapon devices, where certain parts and assemblies are isolated and do not become part of a major circuit until the weapon is actually ready to be fired. Hence, they cannot be protected from electrostatic damage as easily as the integrated circuits in computers as mentioned above.